In the field of data processing, data are stored on and retrieved from a data storage medium. Typically, the storage medium has at least one data track on which data are written or read by a recording head, while a servo control mechanism precisely positions the head relative to the track. A common example of the data storage and retrieval system is disk drive which includes at least one data storage disk having a plurality of concentric circular data tracks, a read/write recording head to read or write data on a magnetic track, and a servo control system to cause the recording head to seek to the selected track and then precisely center the head over the track. Appropriately stored servo data provide information for seeking a selected track and for maintaining the recording head centered on the track.
Various types of servo control systems and servo data are employed to carry out the above-mentioned functions. A common problem with the prior servo control systems, particularly those portions which are used to maintain the recording head centered on a selected track, is that they employ analog design concepts requiring additional components such as an automatic gain control to maintain accurate centering of the recording head relative to the track centerline. Furthermore, the prior servo data, particularly that stored for maintaining the recording head centered on the track, are relatively complicated and involved. Another disadvantage of the prior art is that the servo control system and/or servo data usually include additional components, data and algorithms to compensate for the effects of temperature and humidity on the storage medium that cause the track to expand or contract relative to one another. Thus prior art systems are costly and complex.
These problems are addressed generally by the invention entitled, "SERVO CONTROL APPARATUS" described in U.S. patent application, Ser. No. 385,105, filed June 4, 1982 U.S. Pat. No. 4,488,187, invented by Ben Alaimo and owned by an Assignee in common present application, which Application is hereby incorporated by reference as if fully set forth herein.
One problem for disk-shaped data storage media and disk drives arises because the disk-shaped data storage medium is mechanically secured on the drive. Thus the disk may be secured slightly off center causing the disk to wobble slightly as it is turned by the disk drive. This wobble will cause the data tracks to rotate so that they run out from under the recording head during part of the rotation. Thus the problem caused by the wobble is called runout. Runout can be caused when a spindle on the disk drive which is used for alignment of the disk is either not perfectly vertical or misaligned in some manner, or a bad bearing in the disk drive may cause wobble. Further, wobble which causes runout may occur if the disk does not lay perfectly flat on the table of the disk drive.
Thus, there is a need for a servo control apparatus which detects and compensates for runout by positioning of the head in order to minimize the effects of runout.